the more bubbles the better, I use two 4 inch stones per 27 gallon DWC rez. And a one 6 inch in my 14 gallon rez. It doesn't really matter where they end up, but in theory you want the bubbles to be where the roots are. It's just as important to me that the bubbles make current in the water to keep it mixed well. I tend to pull them into the center after I top off a rez.
I use expensive air stones too. But they last forever, you clean them once or twice a year. Expanded silica,
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